In recent years there has been growing interest in the use of polymers for electronic applications. One particular area of importance is organic photovoltaics (OPV). Polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based photovoltaic devices are achieving efficiencies up to 7%.
A class of polymers that is achieving the highest efficiencies in polymer based photovoltaic devices is based on units that have a high quinoidal contribution. Poly(thiophene), for example, has both an aromatic and a quinoidal contribution as shown below:

A quinoidal structure reduces the torsion between adjacent rings, which results in a more planar polymer backbone leading to an extension of the effective conjugation length. It is generally observed in conjugated polymers that an increase in the conjugation length results in a decrease of the bandgap, leading to a higher degree of absorbed incident light.
The quinoidal state can be stabilised by fusing an aromatic ring to the thiophene backbone. The fused ring is only fully aromatic when the backbone is in the quinoidal state. This means there is a strong desire for the polymer to be in the quinoidal state. Previous work has demonstrated that the bandgap can be reduced by using a benzo-fused thiophene, as in poly(isothianaphthene) (1) shown below [see J. Roncali, Chem. Rev., 1997, 97, 173] or by using a thieno[3,4-b]thiophene, as in poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b′]dithiophene) (2) shown below, wherein R′ is an octyloxy group and R″ is a dodecyloxycarbonyl group [see Y. Liang; Y. Wu; D. Feng; S.-T. Tsai; H.-J. Son; G. Li; L. Yu, J. Am. Chem. Soc., 2009, 131 (1), 56-57].

However, poly(isothianapthene) (1), which is either prepared chemically or electrochemically suffers from insolubility due to a lack of solubilising functionality. Soluble alkylated poly(isothianapthenes) have also been discussed by J. Roncali, Chem. Rev., 1997, 97, 173, however, there is no hint to the semiconducting properties of this or similar materials, or that this or similar polymers could be suitable for use in OPV devices.
Regarding polymers based on benzo[1,2-b:4,5-b′]dithiophene units (hereinafter also referred to as “BDT”), like poly(thieno[3,4-b]thiophene-benzo[1,2-b:4,5-b′]dithiophene) (2), these have certain drawbacks with respect to their polymerisation methods as usually employed.
Usually, such polymers are synthesized by an organometallic catalyzed aryl-aryl coupling reaction of the monomeric units. Typically used coupling reactions include e.g. Yamamoto coupling of the monomers with reactive halide groups (see e.g. Yamamoto, T.; Morita, A.; Miyazaki, Y.; Maruyama, T.; Wakayama, H.; Zhou, Z. H.; Nakamura, Y.; Kanbara, T.; Sasaki, S.; Kubota, K. Macromolecules 1992, 25, 1214-1223, and Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224-2225), or Suzuki coupling of the monomers with reactive halide and/or boronic acid or boronic acid ester groups (see e.g. Schlüter, A. D. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1533-1556), or Stille coupling of the monomers with reactive organotin groups (see e.g. Bao, Z.; Chan, W. K.; Yu, L. J. Am. Chem. Soc. 1995, 117, 12426-12435).
However, when preparing polymers from thiophene-containing monomers, like thieno[3,4-b]thiophene-2,6-diyl, via a Suzuki coupling reaction where the thiophene rings are functionalized with boronic acid or boronic acid ester groups, an undesired side-reaction called deboronation can occur to a significant degree [see M. Jayakannan, J. L. J. van Dongen, R. A. J. Janssen, Macromolecules, 2001, 34, 5386]. Deboronation prematurely halts the polymerization and leads to the formation of low molecular weight polymer chains which lack the requiste physical properties. On the other hand, Stille coupling, which was reported in the literature for the preparation of polymers like (2) above, employs the use of highly toxic organotin reagents, which is undesirable for mass production.
Therefore, there is still a need for organic semiconducting (OSC) materials that are easy to synthesize, especially by methods suitable for mass production, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processability, especially a high solubility in organic solvents, and high stability in air. Especially for use in OPV cells, there is a need for OSC materials having a low bandgap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, especially by methods suitable for mass production, and do especially show good processability, high stability, good solubility in organic solvents, high charge carrier mobility, and a low bandgap. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing conjugated semiconducting polymers as described hereinafter. These polymers comprise an acceptor unit based on a group that is formed by introduction of two thiophene units onto a benzene core, to give 4,8-benzo[1,2-b;4,5-b′]dithiophene (4,8-BDT), or derivatives or isomers thereof, which has additional solubilising groups R in 2- and 6-position:
wherein X is S, Se or O, Y is CH, CR′ or N′, R is e.g. alkyl, alkylcarbonyl or alkyloxycarbonyl, and R′ is e.g. alkyl.
When preparing polymers by aryl-aryl coupling of 4,8-BDT monomer units as used in the present invention, the coupling reaction occurs on the benzene ring of the BDT monomers, and not on the thiophene rings as in 2,6-BDT. This is expected to reduce or even avoid the above-mentioned problems observed in the Suzuki coupling of 2,6-BDT monomers. Also, the use of Stille coupling with highly toxic organotin agents can be avoided. The monomers and polymers of the present invention are therefore especially suitable for large scale production. At the same time, they show good processability, high solubility in organic solvents, a low bandgap and a high charge carrier mobility, and are thus promising materials for organic electronic OE devices, especially for OPV devices. M. Pomerantz, J. Wang, S. Seong, K. P. Starkey, L. Nguyen, D. S. Marynick, Macromolecules, 1994, 27, 7478-74853 reports a polymer of benzo[1,2-b:4,5-b]dithiophene-4,8-diylvinylene, which was prepared by a soluble precursor and then thermally converted as a thin film to an insoluble conjugated polymer. However, the final polymer was reported to be insoluble. Also, there was no reported indication that a substituted benzo[1,2-b:4,5-b]dithiophene unit would show promising performance in semiconducting polymers or be useful as an acceptor unit in copolymers for OPV use. Besides, there is no hint to prepare polymers by aryl-aryl coupling reactions of the monomeric units, or the potential problems linked to these reactions, or possible ways how to overcome these problems.
It was found that the polymers according to this invention are suitable for use as OSC materials in electronic devices, especially in OPV cells, as they have good processability and solubility, and at the same time show a high charge carrier mobility, a low bandgap and a high oxidative stability.
In particular it was found that the repeating units according to the present invention are suitable as acceptor units in dono-acceptor polymers, especially for use in bulk heterojunction (BHJ) OPV devices.